Transmission electron microscopy (TEM) is becoming increasingly important in the characterization of semiconductor devices. One such application is in the recent developmental work of integrated semiconductor devices in which several layers of different materials are deposited and some of the layers may be only a few atoms thick. The dimensions of the lateral structures may be less than a fraction of 1 .mu.m. In order to characterize these devices at these very small dimensions, TEM is felt to be necessary. For investigations of the structural variation with depth, it is necessary that the TEM be performed on a vertical cross-section of the device. However, because of the strong absorption of electrons in a solid, the necessary cross sections must have a thickness on the order of 50 nm and certainly no more than 200 nm.
Once well known method of preparing such thin cross-sectional samples involves ion milling a hole through a relatively thin sample. A wafer has fabricated thereon a surface layer structure. Fragments of the wafer are epoxied together with their structured surfaces facing each other. The thickness of the sample is then reduced in the direction extending along the epoxy by means of mechanical polishing. Therefter, the thinned sample is ion milled. The purpose is to form a small hole in the vicinity of the epoxy bond which will expose wedge-like cross sections of the surface structure.
However, this ion milling method suffers from the defect that it not only takes significant time to mill through a thickness sufficient to support a planar sample but also that the location of the perforation is difficult to control. The diameter of the ion beam is typically wider than the desired hole. The hole has a tendency to develop at a local defect within an area of the beam. Therefore, the hole may be centered off the epoxy. Ion milling necessarily involves high vacuum chambers and ion sources. Thus, the ion milling equipment is expensive. Ion milling involves projecting ions of at least a few keV of energy at the material. Such energetic ions not only sputter away the sample material but also produce a certain amount of subsurface damage in the area of the sample to be later analyzed by TEM. A further problem is that many materials, such as CdTe, are prone to damage and artifacts and therefore exotic types of ion milling are required, for example, reactive ion milling using iodine or chlorine ions. Such equipment is very specialized and very expensive.
In order to eliminate some of the problems of ion milling, a further step of dimpling has found widespread use. Dimplers are commercially available from Gatan Corporation of Pleasanton, Calif., VCR Group of San Franciso, Calif., and Southbay Technology of Temple City, Calif. A dimpler will be described in more detail later but can be generally described as a mechanical grinder which forms a spherical depression or dimple in the sample. In use for forming TEM samples, the dimpler forms a dimple in a relatively thick sample of the sort described before, that is, the two fragments epoxied together. The dimple extends to a depth such that the bottom of the dimple is in the area of the epoxy layer and the remaining thickness of the sample below the dimple is relatively thin. Thereafter, the ion milling is performed on the thin sample at the bottom of the dimple.
The dimpling provides two advantages. First, the portion of the sample away from the dimple is relatively thick and thus provides mechanical rigidity so that the minimum thickness at the bottom of the dimple can be made much thinner. Therefore, less ion milling is required to form the hole. Secondly and more importantly, the ion-milled hole will form at the thinnest portion of the sample. Therefore, if the epoxy layer is aligned with the center of the dimple, the hole will most likely form across the epoxy layer and in both of the wafer fragments. Hence, it is much more likely that a usable cross-sectional sample will be obtained for TEM.
Nonetheless, the combination of dimpling and ion milling suffers from many problems. The problems of the expense of ion millers and of compounds which are damaged or are unstable under ion milling remain. A common layered semiconductor structure of great interest involves III-V compounds such as InP. Much difficulty has been experienced in providing high quality cross-sectional samples of InP for TEM.
The dimpler is a relatively simple mechanical device which is inexpensive and easy to operate. In its present form, it significantly improves the utility of ion milling but does not overcome the difficulties and expense inherent in ion milling.
Another conventional method of perforating metal foils and crystalline slices is jet-thinning. In this method, a stream of corrosive fluid is directed at the specimen until perforation occurs. Bromine or chlorine in methanol is well known for polishing planar samples of compound semiconductors and has further been used in jet thinning. An extension of this idea involves electro-polishing when the corrosive action of the fluid is enhanced or supplanted by electrolytic ablation. In any case, jet thinning has never been successfully applied to producing cross-sectional TEM samples.